If gamma-ray bursters are at cosmological distances, they can be used to probe for «femtolenses» extremely small (∼10 −13 -10 −16 M ⊙ ) dark-matter objects. The time delay induced by such a lens would be ∼10 −17 -10 −20 s, approximately equal to the period of a gamma ray. Interference between the two images would induce a characteristic interference pattern as a function of detected frequency. The pattern would be stable on time scales of 1 s, but might slowly drift on time scales of 10 s in response to the relative motion of the lens and source